Abstract

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The goal of this study is to examine the role of Beclin 1 in the function of estrogen receptor alpha (ERalpha) in breast cancer. Cell signaling through the ERalpha is pivotal to the regulation of breast cancer cell growth so that the ERalpha antagonists are standard therapy for this subset of breast cancer. Beclin 1 is an essential mediator of autophagy, the self-eating process, with a critical role in the regulation of cell growth and cell death. Beclin 1 is also a tumor suppressor gene since one allele of Beclin 1 is lost in subsets of breast, prostate, and other tumors. Since estradiol (E2) -dependent MCF-7 tumor growth in immune-deficient mice was reported to be inhibited by Beclin 1 overexpression, we examined the effect of Beclin 1 overexpression on the action of E2 and 2 antiestrogens, raloxifene and 4-hydroxytamoxifen, in ERalpha-positive MCF-7 breast cancer cells. [3H]-thymidine incorporation studies showed that Beclin 1 overexpressing cells (MCF-7.beclin) had a lower proliferative response to E2 compared to cells transfected with vector control (MCF-7.control). At the 24 h time point, there was a 35% increase in [3H]-thymidine incorporation in E2-treated MCF-7.beclin cells compared to untreated control. In contrast, MCF7.control cells showed a 2-fold increase in [3H]-thymidine incorporation due to E2 treatment. While E2-induced growth of MCF-7.control cells was completely inhibited by 500 nM raloxifene or 500 nM 4-hydroxytamoxifen, these concentrations of antiestrogens had no significant effect on the growth of MCF-7.beclin cells. Confocal microscopic studies showed that treatment with E2 caused a re-organization of Beclin 1 in MCF-7.beclin cells. In addition, we found a co-localization of ERalpha and Beclin 1 in MCF-7.Beclin 1 cells. An interaction between Beclin 1 and ERalpha was also evident from the results of co-immunoprecipitation studies. Furthermore, addition of E2 caused a decrease in Akt phosphorylation in MCF-7.beclin cells. Our results reveal an interaction between ERalpha and Beclin 1 in breast cancer cells. This interaction may modulate the function of ERalpha as well as that of Beclin 1. With regards to ERalpha function, we found that Beclin 1 transfected cells were less sensitive to E2-induced growth stimulation as well as to the growth inhibitory effects of antiestrogens, 4-hydroxytamoxifen and raloxifene. Thus, a novel function for Beclin 1 might involve down-regulation of the action of ERalpha and it may thus contribute to resistance of breast cancer cells to antiestrogens. This observation might be useful to fully define and antiestrogen resistance in breast cancer and devise strategies to bypass Beclin 1 interactions with ERalpha.